The present invention relates generally to magnetic thin films for recording information, and, more particularly, to the use of a seed layer for an underlayer of a magnetic thin film.
Magnetic media are widely used in the computer industry. The media can be locally magnetized by a write transducer, or write head, to record and store information. The write transducer creates a highly concentrated magnetic field which alternates direction based on bits of the information being stored. When the local magnetic field produced by the write transducer is greater than the coercivity of the recording medium, then grains of the recording medium at that location are magnetized. The grains retain their magnetization after the magnetic field produced by the write transducer is removed. The direction of the magnetization matches the direction of the applied magnetic field. The magnetization of the recording medium can subsequently produce an electrical response in a read transducer, allowing the stored information to be read.
The computer industry continually seeks to reduce size of computer components and to increase the speed at which computer components operate. To this end, it is desired to reduce the size required to magnetically record bits of information. It is concomitantly important to maintain the integrity of the information as size is decreased, and disc drives which magnetically store information must be virtually 100% error free. The space necessary to record information in magnetic media is dependent upon the size of transitions between oppositely magnetized areas. It is generally desired to produce magnetic media which will support as small of transition size as possible. However, the output from the small transition size must avoid excessive noise to reliably maintain integrity of the stored information. Media "noise", as used herein, compares the sharpness of a signal on readback against the sharpness of a signal on writing.
In a recording medium with a square hysteresis loop, the width of a recorded transition, a, is predicted to be:
a=(M.sub.t td/.pi.H.sub.c).sup.0.5
wherein M.sub.t is remanent magnetization;
t is medium thickness;
d is the distance from the write transducer to the medium; and
H.sub.c is medium coercivity.
The transition widens with M.sub.t as a result of the fact that the magnetic field existing on one side of a transition affects magnetization on the other side of the transition. The transition narrows as H.sub.c is increased, because with high coercivity, the medium can resist the transition broadening due to the neighboring fields. The magnetic field produced by the write transducer, or head field gradient, is sharpest near the pole tips of the head. The transition widens with d and t, due to the fact that a poorer head field gradient is obtained within the medium when the particles are a further distance from the head. Smaller head-to-medium spacing and thinner medium both lead to narrower transitions being recorded. To decrease transition size, it is desired to produce magnetic media which have higher coercivities, while still providing low noise.
Several material parameters influence the ability of a material to magnetize. Shape anisotropy affects the ease of magnetic recording, as particles are more easily magnetized along the long dimension of the particles. Magneto-elastic anisotropy of a material may affect the ease of magnetic recording. Crystalline anisotropy affects the ease of magnetic recording based on the orientation of crystal structures in the material. In thin films, crystalline anisotropy is the primary means of magnetization. In a disc, grains are more easily magnetized along the plane of the disc because the grains have a preferred crystalline orientation for magnetization lying along the plane. The magnetization results given herein are along the plane of the disc.
Magnetic thin films are a particular type of magnetic medium which are commonly used in computer applications. Thin film media typically consist of a layer or film of a magnetic substance deposited over a substrate. The magnetic substance may be a cobalt based alloy, and the substrate may be a nickel-phosphored aluminum or may be silicon or glass based. A relatively non-magnetic underlayer such as chromium may be used between the magnetic film and the substrate.
To enhance the durability of the disc, a protective layer of a very hard material is applied over the cobalt alloy film. A typical protective layer is an overcoat of sputtered amorphous carbon about 100 to 300 Angstroms thick. The overcoat surface is usually lubricated to further reduce wear of the disc due to contact with the magnetic head assembly. The lubricant is typically applied evenly over the disc in a molecularly thin film having a thickness from 10 to 50 Angstroms. Perfluoropolvethers (PFPEs) are currently the lubricant of choice for thin film recording media.
When applied as a thin film,. the crystal structure of the magnetic layer depends firstly on the composition of the magnetic layer, but also depends on the deposition conditions and processes. Cobalt based alloys have been sputtered onto substrates with chromium underlayers to produce media with coercivities in the range of 1800-2900 Oersteds. The coercivities and media noise can be affected significantly by optimizing the deposition processes.
Coercivities and media noise can also be affected significantly by the composition and microstructure of the underlayer. The initial grain growth of the magnetic layer is dependent on the underlying grain structure of the underlayer. Chromium underlayers have often been used to foster a microstructure in a Cobalt-based magnetic layer with high coercivity and low noise. Underlayers of other materials have also been reported, such as NiAl, Mo, W, Ti, NiP, CrV and Cr alloyed with other substitutional elements. However, only a few of the underlayers actually perform well, and the most successful underlayer has been pure chromium. It is believed that the BCC crystalline structure of Cr underlayers promote grain-to-grain epitaxial growth of the HCP microstructure of cobalt-based thin films, providing a magnetic layer with small grains with in-plane c-axis texture. The chromium underlayer may be applied in a single deposition, or may be applied in two separate deposition steps, in which the first layer may be referred to as a chromium seed layer for the second layer.